1. Field
This invention relates to pulse laser measurement and characterization.
2. Background
Autocorrelators have been widely used to measure the pulse width of ultrafast laser pulses. An autocorrelation method includes splitting a laser beam into two, delaying one of them in time/space, and recombining them for time-correlation characterization. A rapid scanning autocorrelation detector device disclosed in U.S. Pat. No. 4,406,542 was invented in 1983. That device includes as an essential part a glass block which changes the delay of the light beam pulse while the glass block is rotating. In that device the optical path length of the light beam inside the rotating glass block changes as the glass block rotates. The dispersion inside the glass is proportional to the travel distance and the spectrum width of the light beam. The device works well with pico-second or nano-second laser pulses, which usually have narrow spectrum bandwidth. However, a 100 femto-second (fs) laser pulse has a spectrum width of about 10 nanometer (nm). There will be considerable distortion if that device is used to measure a 100 fs laser, which is a disadvantage of such device. The same issue exists in other designs of autocorrelators, such as are disclosed in U.S. Pat. No. 4,190,366 and U.S. Pat. No. 4,265,540.
A second disadvantage of the device of U.S. Pat. No. 4,406,542 is that the beam intensity of the delay line changes when the glass block is rotating. It is because the transitivity of the dielectric depends largely on the incident angle. Thus, the measured autocorrelation trace is incorrect.
A third disadvantage in the devices of the three above-mentioned US patents is that the measurement range is fixed after the device is made. This disadvantage becomes a problem when a device having 100 ps scan range is trying to measure a 100 fs pulse which has low repetition rate, such as 1 kHz. The device will not pick up enough points (pulses) in one scan. Thus, the measurement becomes impossible.